Metallic surface coatings are commonly applied to electronic devices and decorative objects to provide corrosion protection and other desired functional properties. Electronic devices comprising copper or copper alloy substrates are typically coated with metallic surface coatings which provide corrosion protection, high surface contact conductivity, and wear resistance. The metallic surface coatings typically comprise precious metals, in particular silver and gold, which provide superior corrosion protection.
For example, in printed circuit board manufacture, a thin layer of silver may be deposited over copper circuitry as a solderability preserver. The silver is generally deposited by an immersion displacement plating, in which silver ions present in the plating composition come into contact with and are reduced by surface copper atoms, according to the following reaction:Cu(s)+2Ag+(aq)=>Cu2+(aq)+2Ag(s).The reduction-oxidation reaction reduces silver ions to silver metal and forms an adhesive silver layer over the copper substrate. The process is self-limiting in that once the copper surface is covered with a layer of silver, copper atoms are no longer accessible to reduce additional silver ions. Typical thicknesses of silver immersion displacement films over copper can be between about 0.05 and about 0.8 microns. See, for example, U.S. Pat. Nos. 5,955,141; 6,319,543; 6,395,329; and  6,860,925, the disclosures of which are hereby incorporated by reference as if set forth in their entireties.
In the manufacture of copper lead frames and connectors and as an alternative finish in PCB manufacture, gold may be applied as a metallic surface coating over copper substrates for corrosion resistance and increased wear resistance. Typically, gold is not deposited directly on the copper substrate, but rather on an intervening base metal underlayer. The base metal underlayer, typically electrolessly deposited nickel, is deposited on the copper or copper alloy substrate. The base metal serves as a diffusion barrier. The precious metal overlayer, such as gold, palladium, or alloys thereof, is then deposited, typically by an immersion displacement method, over the base metal underlayer coating. The precious metal overlayer provides corrosion resistance, wear resistance, and high conductivity. In the conventional electroless nickel-immersion gold method (commonly referred to as ENIG), an electrolessly deposited nickel underlayer increases the hardness of an immersion plated gold overlayer. This metallic surface is commonly referred to as “nickel-hardened gold” or simply, “hard gold.” Variations on these coatings involve base metal alloy underlayers, precious metal alloy overlayers, and metallic surface coatings comprising two or more base metal underlayers and/or two or more precious metal overlayers.
An obvious disadvantage to the use of precious metals such as gold and palladium is cost. A cost effective connector uses a precious metal coating layer which is as thin as possible, without sacrificing the desired functional properties. Accordingly, the industry typically employs precious metal layer on the order of about 1.0 μm thick on electronic connectors. Thinner layers suffer from the disadvantage of highly increased porosity in the coating. Over time in service, the thin layers having a high degree of  porosity are ineffective against base metal and copper diffusion to the surface. In a corrosive environment, the exposed base metal and copper will corrode and the corrosion product(s) can migrate onto the coating surface and deteriorate the surface contact conductivity. Moreover, a thin precious metal layer can wear off during application and shorten the connector's useful lifetime.
A particular problem observed with immersion-plated precious metal coatings, e.g., silver and gold, is creep corrosion of copper salts at certain bare copper interfaces between copper and precious metal. For example, immersion silver displacement plating processes may not sufficiently coat copper wiring in PCB, particularly at plated through holes and high aspect ratio blind vias. Corrosion at these locations manifests itself as an annular ring surrounding the vias and plated through holes.
Moreover, silver is susceptible to sulfidation by reduced sulfur compounds (e.g., hydrogen sulfide) present in the environment, particularly at paper processing plants, rubber processing plants, and high pollution environments. Sufficient sulfidation of silver can result in localized pores, which may expose copper to the environment. Humidity and environmental pollutants can oxidize and sulfidize the copper, forming copper salts that may creep through pores in the silver layer.